Vacuum chucks are devices that are used to secure semiconductor (e.g., monocrystalline silicon) wafers during processing by, for example, photolithographic stepper machines. Conventional vacuum chucks typically include a flat, air permeable support surface positioned over a vacuum chamber. During operation, a wafer is placed on the support surface, and air pressure inside the vacuum chamber is reduced by way of a centralized vacuum pump. The low pressure inside the vacuum chamber pulls the wafer against the support surface such that a lower surface of the wafer blocks air flow through the support surface, whereby the wafer is securely held in an extremely flat position on the vacuum chuck. The vacuum chuck is then passed through one or more processing stations in which the upper (exposed) surface of the wafer is subjected to one or more fabrication processes (e.g., the deposition of a resist layer, photolithographic exposure of the resist layer, development of the exposed photoresist layer, and removal of exposed/unexposed photoresist material). Once processing is completed, pressure inside the vacuum chamber is increased to facilitate removal of the processed wafer, a new (unprocessed) wafer is mounted and secured to the vacuum chuck in the manner described above, and then the fabrication processes are repeated.
A problem with conventional vacuum chuck arrangements is that they are not conductive to high volume wafer processing. Conventional systems sometimes use a succession of vacuum chucks connected by pressure hoses to a central vacuum source, with the wafers passed from one vacuum chuck to the next at each step of the process. A problem with this approach is that it requires frequent mounting and dismounting of the delicate wafers from the various vacuum chucks, which results in increasing cumulative positional error, increased losses due to wafer damage, and can also produce undesirable temperature variations. The mounting and remounting problem can be avoided by using a single vacuum chuck to carry each wafer through several processing stations, but this would greatly decrease processing throughput. A “conveyor belt” series of vacuum chucks could be arranged to move multiple wafers through the system, but this approach is greatly complicated by the hoses and wiring connected to each vacuum chuck. In some cases a rotary stage with vacuum chucks fixed to it is used. The plumbing is simplified by a single rotary joint in the line. However, this approach does not allow for the linear motion required for extruding straight lines on a wafer passing under a print head, as is required by the solar cell fabrication process described below. Nor does it allow a large number of steps to the process without a prohibitively large table.
An additional problem arises when a fabrication process requires that the wafer be cooled. One such process is described below in the fabrication solar cell devices having extruded high aspect ratio gridlines, wherein the extrusion process requires rapid cooling of the extruded materials when they contact the solar cell wafer. A practical approach to achieving this cooling function is to cool the support surface of each vacuum chuck. Utilizing conventional practices similar to those used to produce vacuum pressure, such cooling would be achieved by providing a central cooling system, and passing the coolant to the various vacuum chucks by way of associated plumbing. However, such as solution would greatly complicate the already difficult process of coordinating the movement of the vacuum chucks without tangling the coolant distribution plumbing.
What is needed is a method and apparatus that facilitates the efficient, high volume production of solar cells and having high aspect ratio extruded gridlines. In particular, what is needed is a vacuum chuck production system that both reliably holds and cools solar cell wafers during the extrusion of high aspect ratio gridlines in a way that avoids the wiring and plumbing problems associated with conventional vacuum chuck systems.